I just got 3 mono's yesterday, about 4-5", they are on pellets and healthy.  Right now they are a little "Pale" in color.  At what age do they start to gain color?
My mono's did not really show great color until they were about 8" long. Houdini is right on with diet and water quality as well. Give them time and they'll be lookers.
Once they hit 12" they will really start to bloom. Your on the right track w/ the pellets. But water quality is number 1 for cichla...as well as most fish.